1. Field of the Invention
The present invention relates to ships and watercraft having improved lift to drag ratios, and more particularly to hull structures which can be used as independent submerged vessels and/or as a submerged body which is part of a vessel that operates at sea level.
2. Background of the Invention
In recent years interest in the use of small waterplane area ships (SWAS vessels) has substantially increased because such vessels have improved hydrodynamic stability, low water resistance and minimal ship motion. Generally such vessels have at least one waterline located below its design draft with a waterplane area that is significantly larger than the waterplane area at its design draft. One form of such vessels is known as a small waterplane area twin hull vessel (a SWATH vessel) which generally consists of two submerged hulls, originally formed of uniform cross-section, connected to a work platform or upper hull by elongated struts which have a cross-sectional area along any given waterplane area that is substantially smaller than a waterplane area cross-section of the submerged hulls. Thus, at the design waterline such vessels have a small waterplane area.
The interest in such vessels has increased in large part because of the development work conducted by the assignee of this application, Pacific Marine Supply Co., Ltd. A variety of such vessels have been produced using twin submerged hulls or a plurality of submerged hulls, such as shown, for example, in U.S. Pat. No. 5,433,161. In the course of the development work for these vessels, further improvements have been made and a so-called Mid-Foil SWAS vessel was developed, as disclosed in U.S. Pat. No. 5,794,558. Such vessels use a lifting body or buoyancy foil which extends transversely of the main hull of the vessel and is submerged during operation. The foil is not a hydroplane foil but a lifting body which produces large hydrostatic and hydrodynamic forces. In the course of continuing development work, the particularized shape of such lifting bodies was studied in detail in order to improve the operation of such vessels.
More specifically, it has been found that the submerged bodies of marine vessels, when operated at shallow submergence depths, such as is the case for SWAS and Mid-Foil vessels, can be adversely effected by the displacement of the free water surface caused by the body's volume and dynamic flow effects. The interaction of that displacement of the free surface relative to the body's shape has not been adequately accounted for in the prior art structures. It is believed that this inadequacy of existing prior art submerged bodies for marine vessels is the result of the fact that submerged and semi-submerged marine vessels have historically been designed to operate at great depths relative to their underwater body thickness, as with submarines or hydrofoils.
A typical submarine is essentially a body of revolution-shaped hull which has three dimensional waterfowl about it, but which is designed to operate normally several hull diameters or more below the free water surface. Thus, the displacement of the free surface of the water by operation of the hull at such depths is minimal and does not effect the operation of the body. On the other hand, hydrofoils are simply submerged wings with predominately two-dimensional flow and are designed typically to produce dynamic lift as opposed to buoyant or hydrostatic lift.
The displacement of water at the free surface by a submerged body is detrimental to a marine vessel's hydrodynamic performance with the impact varying as a function of the body's shape, submergence depth, speed and trim. For example, the free surface effects can significantly reduce lift in the body or even cause negative lift (also referred to as sinkage) to occur.
Resistance to movement through the water by free surface effects is generally greater than if the submerged hull were operating at great depths; and pitch movements caused by the displacement of the free water surface vary with speed and create craft instability. With the advent in recent years of marine vehicles (such as the SWAS, SWATH, and Mid-Foil vessels) which use a shallowly submerged body the detrimental effects of free surface water displacement on submerged hulls has been recognized.
To date, submerged displacement craft hull body shapes are generally cylindrical or tear-drop shaped bodies of revolution. The simplest variations are bodies with generally elliptical cross-sections, such as are shown, for example, in U.S. Pat. Nos. 4,919,063 or 5,433,161. Others are simply shaped in a manner similar to an airplane wing, as shown for example, in U.S. Pat. No. 3,347,197. On the other hand, hydrofoil dynamic lift shapes are generally thin-foils with little or no buoyancy and symmetric foil sections having straight leading and trailing edges. In plan these foils are generally straight, or are swept forward or rearwardly and/or are trapezoidal in shape. Additionally, they can have dihedral or anhedral canting from the horizontal. It has been found that the performance of vessels using these shapes is adversely effected by the displacement of the free surface of the water above the bodies during operation of the vessel.
It is an object of the present invention to provide a submerged hull body which can be employed on various marine vessels to maximize performance of the vessel by creating a high lift to drag ratio (L/D), i.e., low drag, at operational speed.
Another object of the present invention is to provide a submerged hull body for use on various marine vessels which improves performance of the vessel at operational speed while creating a dynamically stable vessel.
Yet another object of the present invention is to provide a submerged hull body for use on various marine vessels or as an independent vessel which has optimal performance at operational depth with a high lift to drag ratio, dynamic stability and which is relatively easy to construct.
A further object of the present invention is to adapt these improved submerged hulls to a variety of watercraft.
Yet another object of the present invention is to produce a submerged hull shape which is adapted to be used as the submerged portion of a vessel whose visible hull operates at or about sea level but which also can operate as an independent vessel at submerged depths.
Another object of the present invention is to provide a submerged hull shape adapted to be used on catamarans, monohulls, multihulls, hydrofoils, and displacement vessels to supplement the vessel's buoyant and dynamic lift with minimal drag.
A further object of the present invention is to provide a submerged hull shape which is stable in operation at low speeds.
Yet another object of the present invention is to provide a submerged hull which has improved sea-keeping ability throughout its range of speed.
A still further object of the present invention is to provide improved propulsive efficiency for submerged vessels.
Yet another object of the present invention is to improve the lift to drag ratio (L/D) for a submerged hull throughout its range of speed in order to improve payload and speed, and/or reduce power necessary to operate the vessel.
Yet another object of the present invention is to reduce the wave making and wash of a vessel.